import os
import sys

def which(cmd, mode=os.F_OK | os.X_OK, path=None):
    """Given a command, mode, and a PATH string, return the path which
    conforms to the given mode on the PATH, or None if there is no such
    file.

    `mode` defaults to os.F_OK | os.X_OK. `path` defaults to the result
    of os.environ.get("PATH"), or can be overridden with a custom search
    path.

    """

    # Check that a given file can be accessed with the correct mode.
    # Additionally check that `file` is not a directory, as on Windows
    # directories pass the os.access check.
    def _access_check(fn, mode):
        return (os.path.exists(fn) and os.access(fn, mode)
                and not os.path.isdir(fn))

    # Check files in "dir" for a match in our list of OK files.
    def _check_files(dir, mode):
        for thefile in files:
            name = os.path.join(dir, thefile)
            if _access_check(name, mode):
                return name
        return None

    is_windows = sys.platform == "win32"
    if is_windows:
        # PATHEXT is necessary to check on Windows.
        pathext = [ext.lower() for
                ext in os.environ.get("PATHEXT", "").split(os.pathsep)]
        # See if the given file matches any of the expected path extensions.
        # This will allow us to short circuit when given "python.exe".
        # If it does match, only test that one, otherwise we have to try
        # others.
        cmd_lower = cmd.lower()
        if any(cmd_lower.endswith(ext) for ext in pathext):
            files = [cmd]
        else:
            files = [cmd + ext for ext in pathext]
    else:
        files = [cmd]

    # If we're given a path with a directory part, look it up directly rather
    # than referring to PATH directories. This includes checking relative to the
    # current directory, e.g. ./script
    dirname = os.path.dirname(cmd)
    if dirname:
        return _check_files(dirname, mode)

    if path is None:
        path = os.environ.get("PATH", os.defpath)
    if not path:
        return None
    path = path.split(os.pathsep)

    if is_windows:
        # The current directory takes precedence on Windows.
        if not os.curdir in path:
            path.insert(0, os.curdir)

    seen = set()
    for dir in path:
        normdir = os.path.normcase(dir)
        if not normdir in seen:
            seen.add(normdir)
            thefile = _check_files(dir, mode)
            if thefile:
                return thefile
    return None


if __name__ == "__main__":
    args = sys.argv[1:]
    if not (1 <= len(args) <= 2):
        sys.exit("Args: command [path-string]")
    print(which(args[0], path=args[1] if len(args) > 1 else None))